Ultrasound imaging has been used to determine information about the interior characteristics of an object or a subject. Generally, an ultrasound imaging system includes a transducer array and components for at least generating and transmitting ultrasound waves, receiving echoes or reflected waves, processing the received signal, generating images, and displaying the images. The transducer array may include a one-dimensional (1D) array of elements or a two-dimensional (2D) transducer array of elements.
For three dimensional (3D) imaging with a 2D transducer array, the elements can be addressed element-wise where each element is individually addressed. Alternatively, the elements can be addressed group-wise, e.g., using row-column addressing. In a configuration in which each element is individually addressed, an N×N array of elements would require N×N (or N2) electrical connections and channels to fully address the array. With row-column addressing, an N×N array of elements can be operated using 2N electrical connections and channels to fully address the array.
As such, the row-column addressing approach can simplify fabrication of the transducer array, for example, due to the reduced number of electrical interconnects to the transducer array (e.g., from N×N to 2N). Furthermore, the data bandwidth requirements are also reduced. Unfortunately, the row-column addressing approach may introduce ghost artifact in the images, for example, because of the significant element height, the lack of an acoustic lens, and the lack of electronic control along the length of the row/column elements.
A non-limiting example of row-column addressing of a 2D transducer array 100 is shown in FIGS. 1 and 2. In FIG. 1, each 1D array 102, 104, and 106 of elements (three elements in the illustrated embodiment) in a first direction (y in the illustrated embodiment) respectively is excited with a single pulse 108, 110, and 112 in transmit. In FIG. 2, a single signal 202, 204, and 206 respectively is produced for each 1D array 208, 210, and 212 of elements (three elements in the illustrated embodiment) in a second different direction (x in the illustrated embodiment, where x is transverse to y) in receive.
FIGS. 3, 4, 5, 6, and 7 provide an example of ghost artifact originating during both receive and transmit in connection with the 1D arrays 208-212 of FIG. 2. For sake of brevity, this example is described with respect to the array 208 in receive. However, the same edge effect arises in transmit due to the receive/transmit reciprocity of the sound field. In FIG. 3, when an emitted waveform is reflected by a point scatterer 304, the reflected wave will have the shape of a sphere 300. At a first time 306 thereafter, the reflected spherical wave 300 intersects only a first subset 308 of elements of the array 208 of the 1D array. FIG. 4 shows the response 402 of the 1D array and the corresponding output signal 404, which is a sum of the measured signals, which are approximately in phase, producing a strong output signal.
Returning to FIG. 3, subsequently, at time 310, the reflected wave 300 intersects with a second subset 312 of elements of the array 208. Where the amplitude of the transmit waveform is symmetric around zero, which it typically is in at least medical ultrasound, the integration of the output is close to zero. FIG. 5 shows the response 502 of the array 208 and the corresponding output signal 504 at time 310. Returning to FIG. 3, subsequently, at time 314, the reflected wave crosses edges 316. FIG. 6 shows the response 602 of the array 208 and the corresponding output signal 604 at time 314.
From the above, during receive, a point scatterer will receive three pressure waves: one main wave and one from each edge of the 1D array. Each of these three waves is reflected and each generates three signals during receive: One main, and one at each edge of the 1D array. Thus, a total of nine signals are generated from one point scatterer. However, only one of these is of interest (the main echo, i.e. the shortest distance from the array to the point scatterer). The remaining eight echoes are artifacts that, in general, are seen as ghosts (three pairs and two single).
Where the point scatterer is located directly above the center of the 1D array (the scenario shown in FIG. 3), the number of ghosts collapses to two (each containing four of the eight artifacts). This is shown in FIG. 7, which shows a main wave 702, and two ghost waves 704 and 706. In the example shown in FIG. 3-7, row-column addressing results in a main lobe at the center at 0 dB, and, beneath it, two ghosts as a result of edge effects on the long row and column elements. Unfortunately, the ghosts introduce artifact, degrading image quality.